1. Field of the Invention
The present invention relates to a voltage detecting circuit for detecting a state of voltage, such as a power supply voltage detecting circuit.
2. Description of the Related Art
A power supply voltage detecting circuit is widely used in various applications, such as detecting the life of a battery or detecting a capacitor power supply voltage in a backup mode state. The power supply voltage detecting circuit is typically incorporated into a micro computer mainly for portable devices. Moreover, the power supply voltage detecting circuit can also be used for the generation of a reset signal for initialization at power-on or generation of a reset signal for prevention of system runaway when the power supply voltage changes.
A configuration and operation of a conventional power source voltage detecting circuit 10 will be described with reference to FIG. 1. The power source voltage detecting circuit 10 includes a detection voltage generating circuit 11, a reference voltage generating circuit 12, and a comparison circuit 13. The detection voltage generating circuit 11 generates a detection voltage 11a for monitoring a power supply voltage. The reference voltage generating circuit 12 generates a reference voltage 12a which is constant and independent of the power supply voltage. The comparison circuit 13 compares the detection voltage 11a with the reference voltage 12a and outputs the result of the comparison as a comparison circuit output signal 13a. A power supply voltage dividing circuit may, for example, be used as the detection voltage generating circuit 11. A bandgap reference circuit may, for example, be used as the reference voltage generating circuit 12.
FIG. 2 shows signal levels of a power supply voltage V, the detection voltage 11a, the reference voltage 12a, and the comparison circuit output signal 13a with respect to time. In FIG. 2, when the detection voltage 11a is higher than the reference voltage 12a, the comparison circuit 13 outputs the comparison circuit output signal 13a at the same level as the power supply level. The detection voltage 11a is designed to be proportional to the power supply voltage V.
As is seen from FIG. 2, whether the detection voltage 11a is higher than the reference voltage 12a depends on the absolute value of variation in the power supply voltage V. When the detection voltage 11a is higher than the reference voltage 12a, the output of the comparison circuit 13 goes to a HIGH level and outputs the comparison circuit output signal 13a which has the same level as that of the power supply voltage V. When the detection voltage 11a is lower than the reference voltage 12a, the comparison circuit 13 outputs the comparison circuit output signal 13a having a LOW level. This operation allows determination if the power supply voltage V has the HIGH level or the LOW level with respect to a predetermined level.
The power source voltage detecting circuit 10 continuously compares the detection voltage 11a with the reference voltage 12a. The comparison circuit output signal 13a is continuously output from the comparison circuit 13. In other words, in the power source voltage detecting circuit 10, all of the detection voltage generating circuit 11, the reference voltage generating circuit 12, and the comparison circuit 13 are continuously in operation so as to detect the power supply voltage. For this reason, there is a problem that a power supply current required to operate these circuits flows continuously.
A plausible way to solve such a problem is to interrupt the power supply current required to operate the circuits if the comparison circuit 13 outputs the comparison circuit output signal 13a. However, this is not practical since the output of the comparison circuit 13 would also be interrupted. Alternatively, the power supply current required to operate the circuits may be reduced. In this case, however, another problem arises such that the circuits are easily affected by temperature fluctuation and variation of their elements, thereby reducing the detection accuracy.
According to one aspect of the present invention, a voltage detecting circuit for detecting a state of a first voltage, includes a detection voltage generating circuit for generating a detection voltage depending on the first voltage; a reference voltage generating circuit for generating a reference voltage; a comparison circuit for comparing the detection voltage with the reference voltage and outputting a result of the comparison as a detection signal; and a control circuit for controlling at least one of the detection voltage generating circuit, the reference voltage generating circuit, and the comparison circuit so that at least one of these circuits operates intermittently.
In one embodiment of this invention, the control circuit includes an oscillation circuit for outputting a clock signal.
In one embodiment of this invention, the voltage detecting circuit further includes a frequency dividing circuit for dividing the clock signal output from the oscillation circuit by a factor of N, wherein N is a natural number.
In one embodiment of this invention, the circuit operating intermittently is the comparison circuit.
According to another aspect of the present invention, a voltage detecting method for detecting a state of a first voltage, includes the step of intermittently comparing a detection voltage as a monitor of the first voltage with a reference voltage, and outputting a result of the comparison as a detection signal.
In one embodiment of this invention, a cycle of the intermittent comparing operation is controlled by a clock signal having a predetermined frequency.
Thus, the invention described herein makes possible the advantages of providing a voltage detecting circuit and method having a low power consumption without a decrease in accuracy of voltage detection.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.